Introduction (1) On the discovery of Desmostylus mirabilis NAGAO from south Sakhalin(p.1) In May 1933, the fossil was discovered by an unknown laborer in Miocene Honto series, at no. 4 dike of Hatsuyuki-sawa, a tributary of the Keton river, Keton, Shisukamachi, Shisuka-gun, South Sakhalin, Japanese former territory. Through excavation in October 1933 and September 1934, the whole skeleton was dug out by late Takumi NAGAOand late Saburo OISHI. The specimen is now in the custody of Department of Geology and Mineralogy, Faculty of Science, Hokkaido University. (2) On the discovery of Paleoparadoxia tabatai (TOKUNAGA) from Gifu Prefecture, Japan (p.1) The whole skeleton of this specimen was discovered and excavated from Miocene Yamanouchi formation by Shigemasa TOMATSU and Mitsuhiko AZUMA, in October 1950, at Inkyoyama, Izumi-machi, Toki city, Gifu Prefecture. This specimen is kept now at the National Science Museum of Tokyo. (3) Establishment of Desmostylus Research Committee (DEREC) (pp.1〜2) For the purpose of preservation and investigation of D.m., DEREC started in 1950. The members of this committee are Hisakatsu YABE (Chairman), Fuyuji TAKAI, Tokio SHIKAMA, Masao MINAT0 and Shoji IJIRI. (4) Progress of study (p.2) The skeletons of D.m. and P.t., were divided into three parts, viz. 1) Skulls 2) Vertebral columns and thoraxes 3) Fore-limbs and hind limbs, the investigation of which was assigned to S. IJIRI, F. TAKAI and T. SHIKAMA respectively. IJIRI finished his allotted work as early as 1954, and the present paper is his first report. I. Description of the skull bones of Din. from South Sakhalin (1) Cranium (pp.3〜4) The cranium is in an imperfect preservation : it lacks anterior parts of maxilla, nassal bone and others, and its mandibulla consists only of molor teeth and ramus of jaw. Moreover, this specimen is severely distorted and hardly any suture can be recognized. So, measurement was taken of the upper, lower, lateral (right side) and hind surfaces of the cranium (Figs. 3-7, Pl. I. 1-4). (2) Skull bones (pp.3〜4) As the sutures of the skull bones are scarcely remained, it is impossible to study each skull bone separately. We are, accordinly, obliged to give only their outlines on the covers of plates. (3) Teeth (pp.5〜6) There was given macroscopic description of P4-M3, 1M-3M ; M2-M3, 2M-3M of D.m. (Pls 7-12). Dental formula of D.m. is inferred to be D=0133/1133 and its dentition to be monophyodonty. II. Description of the skull bones of P.t. from Gifu Prefectures, Japan (1) Cranium (pp.6〜10) The cranium is preserved almost perfectly except a portion of sphenoid bone, palate bone, incisors and premolars, and hyoid bone. Measurement was taken of the upper, lower, lateral (left side) and hind surfaces of the cranium ; and also of lateral and upper surfaces of the mandibulla (Figs. 9-16, P1. III, i-5, P1. IV, 1-4). (2) Skull bones (pp.10〜12) Sixteen skull bones are described macroscopically. (3) Teeth (pp.12〜15, 17) Together with the observation by X-rays, macroscopic descriptions of every tooth of P.t. are given. Hence it is concluded that P.t. is diphyodonty herbivorous and its dental formula is D=3143/ 3143. III. Taxonomy of Desmostylus and Paleoparadoxia from the view point of skull bones (1) Taxonomy of Desmostylus (pp.17〜19) Flaying no intention to go too far into the taxonomical problems, we have followed the term "Desmostylus mi rabilis" in this case, but the skull of D.m. is almost identical with that of Desmostylus hespers. As it was difficult to make comparative antomy on each individual skull bone of D.m., we depended mainly on the teeth comparative anatomy and comparative histology by means of optical- and electron-microscope. Consequently, we have brought out some problems of (i)-(viii) which will be useful for the future study of the taxonomy of Desmostylus. We have come to a new conclusion that, if it is justifiable to give a special

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